Source : WordNet®

sternocleidomastoid muscle
     n : one of two thick muscles running from the sternum and
         clavicle to the mastoid and occipital bone; turns head
         obliquely to the opposite side; when acting together they
         flex the neck and extend the head [syn: {sternocleidomastoid},
          {sternocleido mastoideus}, {musculus
         sternocleidomastoideus}]